This application claims Paris Convention priority of DE 199 29 072.5 filed Jun. 25, 1999 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a device, e.g. an inspection pig, for inspecting conduits made from ferromagnetic materials, such as pipelines, for faults, cracks, corrosion or the like, comprising at least one pulling element, a supporting structure with variable circumference, disposed on the pulling element, and comprising substantially radially disposed supporting arms pivotable about axes disposed perpendicular to the longitudinal central axis of the pulling element, and several permanent magnets disposed on the circumference of the supporting structure for generating a magnetic field, and also having sensors.
So-called inspection pigs are used for inspecting conduits, in particular for transporting water, oil or gas, comprising inspecting means with inspecting elements or sensors, disposed at the outer circumference for inspecting the state of the conduit walls. The sensors can be of various designs. Conventional sensors are i.a. piezo-electrical, electro-acoustic, and electromagnetic sensors such as Hall, stray flux and eddy current sensors.
Different wall conditions or wall thickness reductions, e.g. due to corrosion etc. provide different signals which can be further processed e.g. in an electronic unit.
Conventional inspection pigs for inspecting conduits of different standard widths, or for introducing the inspection pigs via supply lines into the conduit to be inspected, comprise radially expandable supporting structures disposed on a central pulling element with inspecting elements and/or sensors at their circumference. Such supporting structures have a circumference which can vary in dependence on the inner cross-section of the conduit and comprise e.g. several substantially radially disposed supporting arms pivotable about axes disposed perpendicular to the longitudinal central axis of the pulling element (DE 197 46 510 A1, DE 197 46 511 A1).
EP 0 775 910 A1 describes a device for inspecting ferromagnetic materials, in particular conduits, with a radio frequency current coil which serves, in connection with a magnetic field, for excitation or detection of ultra sound waves, wherein the magnetic field is substantially generated by permanent magnets disposed at the circumference of the conduit. An additional magnet arrangement generates a background magnetic field.
These above mentioned devices, based on an electromagnetic measuring principle, have the disadvantage that the magnetic field of the permanent magnets disposed at the circumference of the supporting structure depends on their lateral separation and on the cross-section of the respective conduit, wherein the density of the magnetic field is higher or the magnetic field strength is higher, the smaller the lateral separation between the permanent magnets or the smaller the cross-section of the respective conduit. Consequently, the measuring sensitivity decreases with increasing cross-section of the respective conduit. Moreover, conduits with varying cross-section do not have comparable measuring results and are subject to differing measurement errors.
It is the underlying purpose of the present invention to avoid these disadvantages in a simple and inexpensive fashion.
This object is achieved in accordance with the invention by a device of the above mentioned kind in that at least some of the permanent magnets are associated with a further magnet having a variable magnetic field for strengthening and/or weakening the magnetic field generated by the permanent magnet in dependence on the circumference of the supporting structure or in dependence on the lateral separation between the permanent magnets.
In a device in accordance with the invention, the magnetic field generated by the permanent magnets can be strengthened or weakened by changing the direction and/or the strength of the magnetic field of the magnets associated with the permanent magnets. In this fashion a magnetic field with a substantially constant magnetic field strength can be obtained for conduits of any cross-section irrespective of the lateral separation between the permanent magnets. For reasons of symmetry, one magnet with a variable magnetic field is associated with each permanent magnet.
A first variant of the embodiment provides that the magnet with variable magnetic field is also a permanent magnet which can be turned by means of an actuator for changing the direction of its magnetic field. Turning of the poles of the magnets can thereby move same e.g. into a position aligned with the orientation of the poles of the permanent magnets to optimally increase the magnetic field generated by the permanent magnets for inspecting conduits with large standard width. Conversely, the magnetic field generated by the permanent magnets can e.g. be weakened to a maximum extent if the rotatable magnets and their poles are moved to an orientation opposite to the poles of the permanent magnets to weaken the magnetic field generated by the permanent magnet for inspecting a conduit with small standard width. By turning the magnets, their magnetic field lines can be oriented at an arbitrary angle with respect to the field lines of the magnetic fields generated by the permanent magnets to thereby strengthen or weaken same in a variable fashion.
The actuator can comprise at least one toothed wheel engaging the rotatable magnet, which can e.g. be formed as a shaft which is rotatably disposed and connected to such a toothed wheel for secure mutual rotation. In this case, the rotatable magnet also comprises a toothed wheel connected for secure rotation therewith. Alternatively, the rotatable magnet, e.g. of cylindrical shape, has a toothing at its circumference.
In a preferred embodiment, the actuator can be driven electrically. The actuator can be driven e.g. by an electric motor which communicates with at least one sensor element for detecting the circumference of the supporting structure or the lateral separation between the permanent magnets.
In accordance with a further preferred embodiment, the actuator is mechanically driven. Such a purely mechanical drive has, in particular, the advantage that no additional, in particular, electrical driving means are required. It is therefore very inexpensive and no additional drive or current supply means are required for the arrangement.
In a preferred embodiment, the mechanically driven actuator is driven by at least one helical spring which resiliently biases the supporting arms laterally outwardly at the circumference of the supporting structure and, with lateral approach or withdrawal of the supporting arms, converts the length change associated with its compression or expansion, into a rotary motion of the actuator.
A second variant of the embodiment provides that the magnet of variable magnetic field is an electromagnet, e.g. an induction coil, which can be supplied with a variable current to change the magnetic field strength. In this case, the induction coil magnets can be supplied e.g. with an induction current inducing a magnetic field oriented in the direction of the magnetic field of the permanent magnets to increase same for inspecting a conduit with large standard width. Conversely, the magnetic field generated by the permanent magnets can be weakened by supplying an opposite induction current to the induction coil to weaken the magnetic field generated by the permanent magnets for inspecting a conduit with small standard width. The induction coils preferably communicate with at least one sensor element for determining the circumference of the supporting structure or the lateral separation between the permanent magnets for varying the strength and/or direction of the induction current depending on the cross-section of the conduit.
Each permanent magnet at the circumference of the supporting structure preferably has an associated longitudinally disposed further permanent magnet for generating a magnetic field extending substantially parallel to the longitudinal central axis of the pulling element. This generates a substantially homogeneous magnetic field with substantially parallel field lines about the entire circumference of the pipe section to be inspected. Advantageously, both permanent magnets are each associated with a further magnet having a magnetic field which can be varied in direction and/or strength.
The magnetising units, each formed by a permanent magnet and a magnet associated therewith having a variable magnetic field are preferentially disposed on parallelogram supports each of which pivots on two supporting arms disposed one after the other. This ensures that the magnetic field lines always extend in the same direction with respect to the conduit walls to be inspected, e.g. essentially parallel to the longitudinal central axis of the conduit.
The parallelogram supports are preferably resiliently biased laterally in an outward direction via spring jaws such that the supporting structure automatically adjusts to conduits with various cross-sections.
In a preferred embodiment, the spring jaws accommodate several laterally separated sensors for inspecting the condition of the conduit walls. The sensors are preferably disposed between the spring jaws via spring elements, wherein the respective mutual lateral separations between the sensors are equal for any circumference of the supporting structure. This ensures highest possible measuring accuracy and measuring sensitivity of the device in accordance with the invention in that the magnetising units provide a magnetic field whose strength is substantially independent of the diameter of the conduit and the sensors are each disposed at constant lateral separations to detect the smallest of signals produced by differing wall conditions in the conduit.
Alternatively or additionally, each supporting arm can be radially outwardly biased by a spring force, e.g. by means of a disc spring set.
Each supporting arm and/or each parallelogram support preferably comprises at least one supporting roller for guidance on the inner conduit wall disposed, for the case of the supporting arm, at its outer end in the region of the pivot axis of the parallelogram support.
In a further development of the inventive device, the supporting arms and/or the parallelogram supports are synchronized. In this fashion, the device is also suitable for use in conduits or pipelines comprising branches e.g. Y- or T-shaped branches. During passage of an inspection pig through such branches, those radially outwardly biased supporting arms or parallelogram supports which do not experience a counter pressure from the pipe inner wall due to the branching are held by neighboring supporting arms or parallelogram supports which abut on the pipe wall opposite to the branch, such that the free supporting arms or parallelogram supports cannot spring out radially. In this manner, damage to the inspecting elements, in particular to those disposed on the parallelogram supports, is prevented.
Moreover, the magnetic field required for inspection during passage of the inspection pig through pipe branches, is thereby not impaired. Each pair of neighboring supporting arms and/or each pair of neighboring parallelogram supports are preferably synchronized to ensure an angular difference between the supporting arms relative to the longitudinal central axis of the pulling element of between 1xc2x0 and 5xc2x0, in particular between 2xc2x0 and 3xc2x0.
In a preferred embodiment, the supporting arms of the supporting structure are mounted to the pulling element via a central sleeve which can be fixed to the pulling element. The central sleeve can thereby be fixed to the pulling element in a radially pivotable fashion, e.g. via a ball and socket joint to facilitate passage of the inspection pig through pipe bends.
The invention is described in more detail below by means of preferred embodiments with reference to the drawing.